Crystal oscillator circuits vary in output frequency due to variations in the resonant frequency of the circuit's crystal reference element. These variations to the crystal element are typically caused by changes in temperature which affect the crystal elements resonant frequency. A few techniques which are used in the art for compensating for these crystal variations include: using crystals having good temperature characteristics; using crystal ovens to maintain the crystal's temperature at a substantially constant temperature; and characterizing the crystal over temperature and compensating for these characterized variations by providing a compensation signal to the oscillator circuit which attempts to offset the variations in the crystal, such as discussed in U.S. Pat. No. 4,967,165 by Lee et al.
The major problem with using high quality (e.g., minimal temperature variation crystals) crystals as described above is that high quality crystals tend to be very expensive and still tend to vary a substantial amount over temperature. The use of crystal ovens are not only expensive since they require quite a bit of parts to implement, but they also tend to be impractical for battery powered applications due to their high current drain. Known compensation techniques that compensate for characterized crystal variations do not provide the frequency stability required for modern communication equipment having high specifications.
Present day frequency synthesizers normally utilize varactors to vary the resonant frequency of the crystal oscillator in order to change the phase-locked-loop (PLL) output frequency. Voltage curves corresponding to different temperatures are also typically programmed over the operating temperature range of the synthesizer into the PLL. The programming, in turn, sets current sources which are enabled at certain temperatures and drive the oscillator varactor differently over temperature in order to compensate the circuit for changes in temperature. The problem with the above compensation scheme is that the synthesizer requires the use of a varactor to adjust the oscillator's frequency. The varactor not only adds additional cost to the circuit but also adds it's own tolerances to the final generated frequency.
A need thus exists in the art for a way of eliminating the use of tight temperature characteristic crystal elements and still be able to provide for a crystal oscillator circuit which exhibits extremely small output frequency variations over temperature. A need also exists for a way of eliminating the need for using varactors in the synthesizer's oscillator circuit as the circuit's tuning vehicle.